Seven men have been convicted of a series of sexual offences in the biggest prosecution of a Rotherham grooming gang by the National Crime Agency.

The group targeted five vulnerable schoolgirls, using drugs and alcohol to rape and sexually assault their victims between 1998 and 2005, Sheffield crown court heard.

One girl, aged about 14 at the time, was forced by her parents to have an abortion after being raped by a group of men in a forest. Another told how she had been exploited by “100 Asian men” by the age of 16.

The trial is the biggest prosecution to date under the National Crime Agency’s Operation Stovewood investigation into child sexual exploitation in the South Yorkshire town.

The £90m inquiry, which is examining offences committed between 1997 and 2013, has identified 1,523 potential victims and is investigating allegations against 426 people, of whom 151 are designated suspects.

One of the victims was raped or sexually abused by the defendants over a number of years after meeting one of them, university student Salah Ahmed El-Hakam, when she was 11, the court heard.

She was raped in a derelict house in Rotherham town centre and under a bridge, while other girls were forced to perform sex acts in cars and in a derelict factory on the outskirts of Sheffield.

Mohammed Imran Ali Akhtar, a 37-year-old delivery driver, was described by detectives as the ringleader of the group, all of whom lived in Rotherham or were said to visit the town centre regularly to abuse vulnerable girls.

Akhtar targeted teenage girls from troubled backgrounds, including two sisters who had been effectively abandoned by their parents. He was found guilty of one rape, one charge of aiding and abetting rape, three indecent assaults, one charge of procuring a girl under 21 to have unlawful sexual intercourse with another, and one charge of sexual assault.

The girls would be given alcohol and drugs before being subjected to degrading sexual abuse or violence while being passed around groups of Asian men, the prosecutor, Michelle Colborne QC, said.

The sisters were frequently stopped by police when in cars with the abusers, the court heard, but the exploitation continued. The abuse began when one of them was as young as 11, Colborne said.

“These sisters, like so many others, were easy to exploit because they needed to be loved.”

Akhtar, as well as Tanweer Ali, 37, sexually assaulted one of the girls numerous times between 1998 and 2001 when she was under 16, the court heard. In a police video interview recorded years later, the woman said her phone number was “passed around” groups of men who demanded to meet her for sex.

“I can honestly say that by the age of 16 I had slept with 100 Asian men. Some I didn’t see again,” she told police. “The ones who come and use you for one time are the ones who are hard to remember.”

Paul Williamson, the head of Operation Stovewood, said it was not uncommon for victims to have had so many forced sexual encounters after being groomed. “What we are talking about here is sustained abuse over a number of months or years in some cases,” he said.

Two other defendants, Nabeel Kurshid, 39, and Iqlak Yousaf, 34, were among a number of men who raped a girl in Sherwood forest, Nottinghamshire, when she was about 14 years old.

The jury heard how the vulnerable teenager was “high as a kite” after being given drugs and was warned she would be abandoned in the forest unless she complied with her abusers’ demands.

She became pregnant as a result of the attack and was forced by her parents to have a termination, suffering a “great deal of psychological trauma as a result”, Colborne said. The girl’s parents found out about the exploitation but were unsupportive, verbally abusing their daughter and locking her out when she arrived home late.

A sixth defendant, Asif Ali, 33, was found guilty of three indecent assaults, and the seventh, who cannot be named, was found guilty of two rapes.

An eighth defendant, Ajmal Rafiq, 39, was acquitted of one charge of indecent assault and one charge of false imprisonment.

The defendants remained silent as they were led to the court cells after being told by the judge, Sarah Wright, that they would be remanded in custody and sentenced on 16 November.

So far, 14 people have been convicted under Operation Stovewood. A further 24 suspects have been charged and 68 interviewed under caution. The investigation, comprising 250 officers, has recorded 648 grooming crimes over a 16-year period.